A conventional suction tube for a vacuum cleaner comprises an outer pipe, an inner pipe inserted into the outer pipe, a guiding bush disposed at the outer pipe, a plurality of detent recesses axially provided at the outer surface of the inner pipe, and a locking device disposed between the inner pipe and the outer pipe. The detent recesses contains a locking pin roller which can be inserted into one of the detent recesses. The locking pin roller has a mantle at its top for keeping the locking device in its locking state. The locking pin roller can move upward out of the recess to make the locking device unlocked, when the mantle is removed. As a result, the inner pipe can be moved axially with respect to the outer pipe. Guiding slopes are provided at the front and rear edges of the detent recesses so as to help the locking pin roller be released from the detent recess easily. However, the locking pin roller is liable to escaping from the detent recess along the guiding slopes under an impact arisen between the inner pipe and the outer pipe. In this case, the locking device will fail to work.
To overcome the above drawbacks, Chinese patent No. 01266578.9 provides a telescopic suction tube for a vacuum cleaner, in which an inner pipe of this telescopic suction tube is provided with a plurality of detent recesses. The sidewalls of the detent recesses are perpendicular to the outer surface of the inner pipe. Although such detent recesses make the locking device thereof be capable of withstanding heavier impact, they are likely to crack at a portion between the sidewall of the recess and the bottom thereof perpendicular to the sidewall arisen by a stress generated from the manufacturing thereof. As a result, air leakage will happen to the inner pipe and the vacuum degree in the inner pipe of the vacuum cleaner will be decreased accordingly, when the vacuum cleaner operates.